Water dripping through tundish and not draining away

[pjwilson83]

Hi, complete novice here.
The other day we noticed a brown stain on the kitchen ceiling.
We have an unvented hot water cylinder in the airing cupboard upstairs and when checking this, I noticed that every time the central heating is on, there are some drips of water coming from the nuts/bolts under the white expansion vessel. There is also water flowing through the tundish, sometimes a steady trickle, sometimes slower drips. Sometimes it feels cold, sometimes lukewarm.
This water is not draining away properly so has backed up causing it to overflow out of the tundish and onto the floor, hence the stain on the kitchen ceiling.
I first thought this was due to the drainage pipe on the outside of the house freezing over due to the recent cold snap. However no amount of defrosting has helped.
The temperature has now gone above freezing and the water coming through the tundish is still not draining away. We have had to 'pipette' it out to stop it from overflowing. As such, we can't risk putting the heating on for fear of it overflowing again.
I do have an engineer from SSE Home Services coming out tomorrow to have a look.
My question is, if the drainage pipe is blocked and this is not caused by ice, what else could be causing this?

 

[ShaunCorbs]

It’s frozen externally probably they normally they normally hide them in a waste pipe sleeve you will know what it is when you find it normally around 18” long coming out of the ground no fittings on it

Will take days to unfreeze naturally and tbh should of sorted it a soon as it started dripping also they need servicing yearly

Also did you inform the insurance it’s on a unvented cylinder and the engineer needs to be qualified g3 etc ?

 

[pjwilson83]

It’s frozen externally probably they normally they normally hide them in a waste pipe sleeve you will know what it is when you find it normally around 18” long coming out of the ground no fittings on it

Will take days to unfreeze naturally and tbh should of sorted it a soon as it started dripping also they need servicing yearly

Also did you inform the insurance it’s on a unvented cylinder and the engineer needs to be qualified g3 etc ?

Thanks for the reply.
To be fair, we did call SSE as soon as we noticed the water dripping through the tundish but it's taking them a while to come out to us since they are so busy. I didn't ask if the guy who is coming out is qualified G3 - I assume he will be as I did say that we have an unvented cylinder when I phoned up.
Regarding the annual service, it was due for service a few months ago which I had to cancel due to COVID and I forgot to reappoint so that's my bad.
What do you mean when you say "they normally hide them in a waste pipe sleeve..." - do you mean the external bit of drainage pipe?
If so, I have located this and over the last few days we have tried everything to try and defrost it. We even had a plumber out (while waiting on the guy from SSE Home services) who put his blow torch on the pipe to try and thaw out any ice inside but still nothing.
So my question is, if it's not ice blocking the pipe, what could it be?

 

[ShaunCorbs]

Correct yes to hide them / protect

It’s one of them need to heat for a long time as it’s probably at least 1m thick up or wait for warmer weather to defrost it naturally

It will be frozen